Lithography laser with beam delivery and beam pointing control

ABSTRACT

The present invention provides a modular high repetition rate ultraviolet gas discharge laser light source with a beam delivery to a production line machine. The system includes an enclosed and purged beam path with beam pointing control for delivery the laser beam to a desired location such as the entrance port of the production line machine. Preferred embodiments include equipment for beam attenuation, equipment for automatic feedback beam alignment and equipment for accurate optics module positioning at installation and during maintenance. In preferred embodiments, the production line machine is a lithography machine and two separate discharge chambers are provided, one of which is a part of a master oscillator producing a very narrow band seed beam which is amplified in the second discharge chamber. This MOPA system is capable of output pulse energies approximately double the comparable single chamber laser system with greatly improved beam quality. A pulse stretcher more than doubles the output pulse length resulting in a reduction in pulse power (mJ/ns) as compared to prior art laser systems. This preferred embodiment is capable of providing illumination at a lithography system wafer plane which is approximately constant throughout the operating life of the lithography system, despite substantial degradation of optical components.

The present invention is a continuation-in-part of Ser. No. 10/255,806filed Sep. 25, 2002 now U.S. Pat. No. 6,704,340, Ser. No. 10/233,253filed Aug. 30, 2002 now U.S. Pat. No. 6,704,339, Ser. No. 10/141/216filed May 7, 2002 now U.S. Pat. No. 6,693,939, Ser. No. 10/036,676,filed Dec. 21, 2001 now U.S. Pat. No. 6,882,674, Ser. No. 10/036,727filed Dec. 21, 2001 now U.S. Pat. No. 6,865,210, Ser. No. 10/006,913filed Nov. 29, 2001, now U.S. Pat. No. 6,535,531, Ser. No. 10/000,991filed Nov. 14, 2001 now U.S. Pat. No. 6,795,474, Ser. No. 09/943,343,filed Aug. 29, 2001 now U.S. Pat. No. 6,567,450, Ser. No. 09/854,097,filed May 11, 2001 now U.S. Pat. No. 6,757,316, Ser. No. 09/848,043,filed May 3, 2001, now U.S. Pat. No. 6,549,551, Ser. No. 09/837,150filed Apr. 18, 2001, now U.S. Pat. No. 6,504,860, and Ser. No.09/829,475 filed Apr. 9, 2001 now U.S. Pat. No. 6,765,945, and claimsthe benefit of Provisional Ser. No. 60/443,673 filed Jan. 28, 2003, allof which are incorporated herein by reference. This invention relates tolithography light sources for integrate circuit manufacture andespecially to gas discharge laser lithography light sources forintegrated circuit manufacture.

BACKGROUND OF THE INVENTION Electric Discharge Gas Lasers

Electric discharge gas lasers are well known and have been availablesince soon after lasers were invented in the 1960s. A high voltagedischarge between two electrodes excites a laser gas to produce agaseous gain medium. A resonance cavity containing the gain mediumpermits stimulated amplification of light which is then extracted fromthe cavity in the form of a laser beam. Many of these electric dischargegas lasers are operated in a pulse mode.

Excimer Lasers

Excimer lasers are a particular type of electric discharge gas laser andthey have been known since the mid 1970s. A description of an excimerlaser, useful for integrated circuit lithography, is described in U.S.Pat. No. 5,023,884 issued Jun. 11, 1991 entitled “Compact ExcimerLaser.” This patent has been assigned to Applicants' employer, and thepatent is hereby incorporated herein by reference. The excimer laserdescribed in Patent '884 is a high repetition rate pulse laser.

These excimer lasers, when used for integrated circuit lithography, aretypically operated in an integrated circuit fabrication line“around-the-clock” producing many thousands of valuable integratedcircuits per hour; therefore, down-time can be very expensive. For thisreason most of the components are organized into modules which can bereplaced within a few minutes. An excimer laser used for lithographytypically must have its output beam reduced in bandwidth to a fractionof a picometer. This “line-narrowing” is typically accomplished in aline narrowing module (called a “line narrowing package” or “LNP” forKrF and ArF lasers) which forms the back of the laser's resonant cavity(A line selection unit “LSU” is used for selecting a narrow spectralband in the F₂ laser). The LNP is comprised of delicate optical elementsincluding prisms, mirrors and a grating. Electric discharge gas lasersof the type described in Patent '884 utilize an electric pulse powersystem to produce the electrical discharges, between the two elongatedelectrodes. In such prior art systems, a direct current power supplycharges a capacitor bank called a “charging capacitor” or “C₀” to apredetermined and controlled voltage called the “charging voltage” foreach pulse. The magnitude of this charging voltage may be in the rangeof about 500 to 1000 volts in these prior art units. After C₀ has beencharged to the predetermined voltage, a solid state switch is closedallowing the electrical energy stored on C₀ to ring very quickly througha series of magnetic compression circuits and a voltage transformer toproduce high voltage electrical potential in the rouge of about 16,000volts (or greater) across the electrodes which produce the dischargeswhich lasts about 20 to 50 ns.

Major Advances in Lithography Light Sources

Excimer lasers such as described in the '884 patent have during theperiod 1989 to 2003 become the primary light source for integratedcircuit lithography. More than 2000 of these lasers are currently in usein the most modern integrated circuit fabrication plants. Almost all ofthese lasers have the basic design features described in the '884patent. This is:

-   -   (1) a single, pulse power system for providing electrical pulses        across the electrodes at pulse rates of about 100 to 2500 pulses        per second;    -   (2) a single resonant cavity comprised of a partially reflecting        mirror-type output coupler and a line narrowing unit consisting        of a prism beam expander, a tuning mirror and a grating;    -   (3) a single discharge chamber containing a laser gas (either        krypton, fluorine and neon for KrF or argon, fluorine and neon        for ArF), two elongated electrodes and a tangential fan for        circulating the laser gas between the two electrodes fast enough        to clear the discharge region between pulses, and    -   (4) a beam monitor for monitoring pulse energy, wavelength and        bandwidth of output pulses with a feedback control system for        controlling pulse energy, energy dose and wavelength on a        pulse-to-pulse basis.

During the 1989-2001 period, output power of these lasers has increasedgradually and beam quality specifications for pulse energy stability,wavelength stability and bandwidth have become increasingly tighter.Operating parameters for a popular lithography laser model used widelyin integrated circuit fabrication include pulse energy at 8 mJ, pulserate at 2,500 pulses per second (providing an average beam power of upto about 20 watts), bandwidth at about 0.5 pm full width half maximum(FWHM) and pulse energy stability at +/−0.35%.

Injection Seeding

A well-known technique for reducing the bandwidth of gas discharge lasersystems (including excimer laser systems) involves the injection of anarrow band “seed” beam into a gain medium. In some of these systems alaser producing the seed beam called a “master oscillator” is designedto provide a very narrow bandwidth beam in a first gain medium, and thatbeam is used as a seed beam in a second gain medium. If the second gainmedium functions as a power amplifier, the system is referred to as amaster oscillator, power amplifier (MOPA) system. If the second gainmedium itself has a resonance cavity (in which laser oscillations takeplace), the system is referred to as an injection seeded oscillator(ISO) system or a master oscillator, power oscillator (MOPO) system inwhich case the seed laser is called the master oscillator and thedownstream system is called the power oscillator. Laser systemscomprised of two separate systems tend to be substantially moreexpensive, larger and more complicated to build and operate thancomparable single chamber laser systems. Therefore, commercialapplication of these two chamber laser systems has been limited.

Separation of Lithography Machine from Light Source

For integrated circuit fabrication the lithography machine is typicallylocated separate from the lithography laser light source. The separationis typically 2 to 20 meters. The laser and the lithography machine maybe located in separate rooms. A typical practice is to locate the laserin a room one floor below the lithography machine. The laser beam isultraviolet at wavelengths of about 248 nm for KrF lasers, 193 nm forArF lasers and 157 nm for F₂ lasers. Ultraviolet light especially at theshorter wavelengths of the ArF and F₂ lasers is absorbed by oxygen,therefore it is a well known practice to enclose the laser beam pathbetween the laser and the lithography machine and to purge the enclosurewith a gas such as nitrogen which provides much lower beam attenuationthan air. Included within the enclosure also are a variety of opticalcomponents, including mirrors and lenses, for directing the laser beamto a desired beam entrance port in the lithography machine and forproviding any needed modification to the beam, such as changes incross-sectional profile. The equipment for delivering the laser beam tothe lithography machine is called a beam delivery unit or “BDU” forshort. In the past the BDU has typically been designed and suppliedseparate from the laser light source.

What is needed is a better laser design for a pulse gas discharge laserfor operation at repetition rates in the range of about 4,000 pulses persecond or greater, providing laser light at the entrance port of thelithography machine having beam quality parameters including wavelength,bandwidth, pulse energy, beam pointing angle, beam position andcross-sectional profile needed by the lithography machine.

SUMMARY OF THE INVENTION

The present invention provides a modular high repetition rateultraviolet gas discharge laser light source with a beam delivery to aproduction line machine. The system includes an enclosed and purged beampath with beam pointing control for delivery the laser beam to a desiredlocation such as the entrance port of the production line machine.Preferred embodiments include equipment for beam attenuation, equipmentfor automatic feedback beam alignment and equipment for accurate opticsmodule positioning at installation and during maintenance. In preferredembodiments, the production line machine is a lithography machine andtwo separate discharge chambers are provided, one of which is a part ofa master oscillator producing a very narrow band seed beam which isamplified in the second discharge chamber. This MOPA system is capableof output pulse energies approximately double the comparable singlechamber laser system with greatly improved beam quality. A pulsestretcher more than doubles the output pulse length resulting in areduction in pulse power (mJ/ns) as compared to prior art laser systems.This preferred embodiment is capable of providing illumination at alithography system wafer plane which is approximately constantthroughout the operating life of the lithography system, despitesubstantial degradation of optical components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a layout of a lithography laser system with a beam deliveryunit.

FIGS. 2, 2A and 2B show features of a pulse stretching unit.

FIGS. 3A, 3B, 3C, 3D, 3E, 3F and 3G show features of a relay optics forthe FIG. 1 laser system.

FIGS. 4A, 4B, 4C and 4D show beam delivery configurations.

FIG. 5 is a graph of pulse energy versus charging voltage.

FIG. 6 shows a technique of turning a beam 90 degrees with prisms.

FIG. 7 shows a laser light source with beam delivery to a scanner.

FIGS. 8A-8E show an easily sealing bellows seal.

FIG. 9 demonstrates a feature of a preferred pulse stretcher.

FIG. 10A shows a beam delivery unit.

FIG. 10B shows details of a metrology monitor for monitoring beam angleand beam position.

FIGS. 10C and 10D1-3 show techniques for monitoring pointing error.

FIGS. 10E, F, G and H show test charts demonstrating performance of abeam pointing control system.

FIGS. 11A-11J show features of a prototype BDU unit.

FIGS. 11K-11O show test results using the prototype unit.

FIGS. 12, 12A, B, C, D, E and F show features of a module alignmenttechnique.

FIGS. 13A through 15C show components and features for controlling beampolarization.

FIGS. 16A through 16I show features of a preferred shutter.

FIGS. 17A, B and C show features of a variable beam attenuator.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS First Preferred Embodiment

A first preferred embodiment of the present invention is shown inFIG. 1. In this embodiment a 193 nm ultraviolet laser beam is providedat the input port of a scanner lithography machine 2 such as the one ofthose supplied by Canon or Nikon with facilities in Japan or ASML withfacilities in the Netherlands. In this case the main components of thelaser system 4 are installed below the deck on which the scanner isinstalled. This preferred embodiment includes a MOPA laser system withspecial relay optics, a pulse stretcher and a beam delivery unit 6 whichprovides an enclosed beam path for delivering the laser beam to theinput port of the scanner. The beam delivery unit includes equipment forbeam attenuation, automatic beam alignment with feedback control andspecial alignment features for component alignment during installationand maintenance.

Mopa

This particular laser system includes a master oscillator 8 and a poweramplifier 10 and is a type of laser system known as MOPA system. ThisMOPA arrangement represents an important advancement in integratedcircuit light sources over the prior art technique of using a singlelaser oscillator to provide the laser Light. The master oscillator 8 andthe power amplifier 10 each comprise a discharge chamber similar to thedischarge chamber of prior art single chamber lithography laser systems.These chambers contain two elongated electrodes, a laser gas, atangential fan for circulating the gas between the electrodes andwater-cooled finned heat exchangers. The master oscillator 8 produces afirst laser beam 14A which is amplified by two passes through the poweramplifier 10 to produce laser beam 14B. The master oscillator 8comprises a resonant cavity formed by output coupler 8A and linenarrowing package 8B both of which are described generally in thebackground section and in detail in the referenced prior art patents.The gain medium for master oscillator 8 is produced between two 50-cmtong electrodes contained within master oscillator discharge chamber 8C.Power amplifier 10 is basically a discharge chamber and in thispreferred embodiment is almost exactly the same as master oscillatordischarge chamber 8C providing a gain medium between two elongatedelectrodes but it has no resonant cavity and the gas pressure is higherthan that of the master oscillator. This MOPA configuration permits themaster oscillator to be designed and operated to maximize beam qualityparameters such as wavelength stability, and to provide a very narrowbandwidth; whereas the power amplifier is designed and operated tomaximize power output. For example, the current state of the art lightsource available from Cymer, Inc. Applicants' employer, is a 5 mJ perpulse, 4 kHz, ArF laser system. The system shown in FIG. 1 is a 10 mJper pulse (or mores if desired) easily up in about 30 mJ per pulse, 4kHz ArF laser system producing at least twice the average ultravioletpower with substantial improvement in beam quality. For this reason theMOPA system represents a much higher quality and much higher power laserlight source.

Relay Optics Beam Path

In this preferred embodiment the output beam 14A of the masteroscillator 8 is amplified by two passes through power amplifier 10 toproduce output beam 14B. The optical components to accomplish this arecontained in three modules which Applicants have named: masteroscillator wave front engineering box, MO WEB, 24, power amplifierwavefront engineering box, PA WEB, 26 and beam reverser, BR, 28. Thesethree modules along with line narrowing module 8B and output coupler 8Aare all mounted on a single vertical optical table independent ofdischarge chamber 8C and the discharge chamber of power amplifier 10.Chamber vibrations caused by acoustic shock and fan rotation must beisolated from the optical components.

The optical components in the master oscillator line narrowing module 8Band output coupler 8A are in this embodiment substantially the sonic asthose of prior art lithography laser light sources referred to in thebackground section. The line narrowing module 8B includes a three orfour prism beam expander, a very fast response tuning mirror and agrating disposed In Litrow configuration. The output coupler is apartially reflecting mirror reflecting 20 percent of the output beam forKrF systems and about 30 percent for ArF and passing the remainder. Theoutput of master oscillator 8 is monitored in line center analysismodule, LAM, 7 and passes into the MO WEB 24. The MO WEB 24 Contains atotal internal reflection (TIR) prism and alignment components forprecisely directing the output beam 14A into the PA WEB 26. TIR prismssuch as the one shown in FIG. 3A can turn a laser beam 90 degrees withmore than 90 percent efficiency without need for reflective coatingswhich typically degrade under high intensity ultraviolet radiation.Alternatively, a first surface mirror with a durable high reflectioncoating could be used in place of the TIR prism.

The PA WEB 26 contains a TIR prism 26A as shown in FIG. 3C-F andalignment components (not shown) for directing laser beam 14A into afirst pass through power amplifier gain medium. Alternatively, as above,a first surface mirror with a high reflection coating could hesubstituted for the TIR prism. The beam reverser module 28 contains atwo-reflection beam reversing prism 28A shown in FIGS, 3B-D that likethe one-reflection prism shown in FIG. 3A relies on total internalreflection and therefore requires no optical coatings. The face wherethe P-polarized beam enters and exits the prism 28A is oriented atBrewster's angle to minimize reflection losses, making the prism almost100% efficient.

After reversal in the beam reversing module 28, partially amplified beam14A makes another pass through, the gain medium in power amplifier 10and exits through spectral analysis module 9 and PA WEB 26 as poweramplifier output beam 14B. In this embodiment the second pass of beam14A through power amplifier 10 is precisely in line with the elongatedelectrodes within the power amplifier 10 discharge chamber. The firstpass follows a path at an angle of about 6 milliradians relative to thepath of the second pass and the first path of the first pass crosses thecenter line of the gain medium at a point half way between the two endsof the gain medium. FIGS. 3C and 3D show side and top views of the pathof beam 14A through the power amplifier 10. The reader should note thatthe design and positioning of beam reversing prism 28A must accommodatean angle β and a spatial offset of d as shown in FIG. 3B. In thisembodiment β=6 milliradians and d is equal to 5 mm.

FIGS. 3E (side view) and 3F (top view) show some additional importantfeatures of optics in power amplifier WEB module 26. Note that in theside view, the beam “to” the PA is shown above the beam “from” the PA.This is done so that both beams can be shown in the side view drawing.(Actually both beams are at the same elevation so that the “from” beamwould block the “to” beam if the from beam were shown at the correctelevation.). As shown in FIG. 3F the from beam passes close to TIR prism26, passes Through exit aperture 26C, and is expanded by a factor of 4in the horizontal direction with two beam expanding prisms 26D and exitsto pulse stretcher module 22 (called by Applicants' “OPUS”, for opticalpulse stretcher). Exit aperture 26C as well as other apertures in therelay optics should be considered optional and they may be rep laced bytemporary alignment targets.

Other TIR Prism Considerations

TIR prisms in the MO WEB and PA WEB are preferred over dielectric-coatedfirst surface mirrors because they have no optical coatings, which tendto degrade with extended exposure to high fluence UV radiation. Onedisadvantage of the TIR prisms is unwanted Fresnel reflections thatoccur at the entrance and exit faces. For calcium fluoride material at193 nm, each face reflects about 4% of the Incident beam. If theincident beam is normal to the surface, the unwanted reflections willpropagate back along the path of the incident beam and re-enter the MO.This could interfere with the stable operation of the MO. The problem isavoided by tilting the entrance and exit faces of the TIP prisms byapproximately 1 degree relative to the incident beam. This can beaccomplished by rotation of a 45°-45°-90° TIR prism by 1 degree, inwhich case the deviation angle of the primary beam will change from 90°to either 88° or 92° (depending on the direction of the 1 degreerotation). Alternatively, a 90° deviation angle and 1 tilted faces canbe achieved by using a TIR prism with angles 44°-44°-92° or 46°-46°-88°or 44.33°-45.67°-90°.

The TIR prism 26A in the PA WEB 26 is used very close to an edge of eachof the three optical faces. The optical faces of these prisms must beaccurately polished to within 1 mm or less of the critical edges.

The TIR prisms in the MO WEB 24 and PA WEB 26 will each be alignable intwo degrees of freedom (2 rotations, “tip-tilt”). The MO WEB TIR prismis aligned so that the primary reflected beam is directed to theappropriate location in the PA WEB 26. The PA WEB TIR prism is alignedso that the primary reflected beam is directed to the appropriatelocation in the Beam Reverser 28. Each TIR prism is secured in amechanical mount which allows the tip-tilt adjustments twin outside thesealed module.

The maximum reflected wavefront error is specified as 0.20 wavepeak-valley at 633 nm (i.e., 127 nm) across the clear aperture (13 mm×21mm). The wavefront error across the much smaller beam will besignificantly less, though the exact amount depends on the type ofaberrations present. If simple curvature is the dominant error (as itgenerally is with polished flats), the maximum divergence angle errorintroduced to a beam would be about 0.02 mrad in the vertical direction(and much less in the horizontal direction).

Degradation of the optical coating over life (especially at 193 nm) is aconcern for high reflection dielectric coatings that are more damageresistant than partial reflection or AR coatings may be used. Alsoaiding the goal of long lifetime for this mirror is the fact that thepulse energy is much lower coming out of the MO 8 than coming out of thePA 10. Because the mirror will be used very close to the edge, thecoating may be more susceptible than usual to damage. There may besurface roughness or coating irregularities near the edge thatcontribute to coating failure. The edge of the mirror preferably istested to avoid these potential problems. FIG. 3G shows the spacingissues. In order to direct the beam to the appropriate location in theBeam Reverser module 28, the turning mirror will be aligned in twodegrees of freedom (2 rotations, “tip-tilt”). The minor mount mustinclude adjustments, accessible from outside the sealed module, foraligning the mirror to the required accuracy.

An alternative to the coated mirror 26A is to use an uncoated TIR prisminstead of the dielectric-coated mirror. Such a design would eliminateany concern of coating damage over life.

Alignment Features for Relay Optics

For this tilted double-pass geometry, the beams reflecting from the MOWEB 24 end the Beam Reverser 28 are precisely positioned in the PA WEB26. Alignment features are provided within the PA WEB 26 for properalignment of the MO WEB 24 mirror and the Beam Reverser 28. The featureswould need to reference to the edge of the TIR prism. Preferably thealignment features are apertures, one at the entrance to the PA WEB 26(for alignment the MO WEB prism) and one at the exit (for aligning thebeam reverser 28). The apertures might be permanent or removable. Thesystem should be alignable In the field with the beam path sealed.Preferably the location of the beam with respect to the apertures willbe made visible with some type of 2-D detector array (digital camera). ABeam Analysis Tool called BAT (perhaps with an aperture built-in) may beinserted into the module to inspect the alignment as shown at 36 in FIG.3F.

Beam Expansion Prisms

Coming out of the PA 10, the fluence of the beam is higher than anywhereelse in the system (due to small beam size and high pulse energy). Toavoid having such high fluence incident on the optical coatings in theOPuS module 22, where coating damage could result, beam expansion prismswere designed into the PA WEB 26. By expanding the horizontal beam widthby a factor of 4, the fluence is reduced to ¼ its previous level.

The beam expansion is accomplished using a pair of identical prisms with20° apex angles as shown in FIG. 3G. The orientation of the prisms andbeam path is shown FIG. 3G.

The prisms are made of ArF-grade calcium fluoride and are uncoated. Byutilizing an incidence angle of 68.6° on each prism, anamorphicmagnification of 4.0 is achieved, and the nominal deviation angle of thepair is zero. The total Fresnel reflection loss from the four surfacesis about 12%.

Pulse Stretcher

Integrated circuit scanner machines comprise large lenses which aredifficult to fabricate and costs millions of dollars. These veryexpensive optical components are subject to degradation resulting frombillions of high intensity and ultraviolet pulses. Optical damage isknown to increase with increasing intensity (i.e., light power(energy/time) per cm² or mJ/ns/cm²) of the laser pulses. The typicalpulse length of the laser beam from these lasers is about 20 ns so a 5mJ beam would have a pulse power intensity of about 0.25 mJ/ns.Increasing the pulse energy to 10 mJ without changing the pulse durationwould result a doubling of the power of the pulses to about 0.5 mJ/nswhich could significantly shorten the usable lifetime of these expensiveoptical components. The Applicants have avoided this problem byincreasing substantially the pulse length from about 20 ns to more than50 ns providing a reduction in the rate of scanner optics degradation.This pulse stretching is achieved with pulse stretcher unit 12 as shownin FIG. 1. An enlarged view showing the beam paths though pulsestretcher 12 is shown in FIG. 2. A beam splitter 16 reflects about 60percent of the power amplifier output 10 beam 14B into a delay pathcreated by four focusing mirrors 20A, 20B, 20C and 20D. The 40 percenttransmitted portion of each pulse of beam 14B becomes a first hump 13Aof a corresponding stretched pulse 13 shown in FIG. 2B of beam 14C. Thestretched beam 14C is directed by beam splitter 16 to mirror 20A whichfocuses the reflected portion to point 22. The beam then expands and isreflected from mirror 20B which converts the expanding beam into aparallel beam and directs it to mirror 20C which again focuses the beamagain at point 22. This beam is then reflected by mirror 20D which likethe 20B mirror changes the expanding beam to a light parallel beam anddirects it back to beam splitter 16 where 60 percent of the firstreflected light is reflected perfectly in line with the firsttransmitted portion of this pulse in output beam 14C to become most ofhump 13B in pulse 13 as shown in FIG. 2B. The 40 percent of thereflected beam transmits beam splitter 14 and follows exactly the pathof the first reflected beam producing additional smaller humps instretched pulse 13. The result is stretched pulse 14C which is stretchedin pulse length from about 20 ns to about 50 ns. The stretched pulse 14Cis plotted as intensity vs. time in FIG. 2B and can be compared with theshape of the power amplifier output pulse 14B which is similarly plottedin FIG. 2A.

The stretched pulse shape with this embodiment has two largeapproximately equal peaks 13A and 13B with smaller diminishing peaksfollowing in time the first two peaks. The shape of the stretched pulsecan be modified by using a different beam splitter. Applicants' havedetermined that a beam splitter reflecting about 60 percent produces themaximum stretching of the pulse as measured by a parameter known as the“time integrated square” pulse length or “t_(IS)”. Use of this parameteris a technique for determining the effective pulse duration of pulseshaving oddly shaped power vs. time curves. The t_(IS) defined as:

$t_{IS} = \frac{( {\int{{I(t)}{\mathbb{d}t}}} )^{2}}{\int{{I^{2}(t)}{\mathbb{d}t}}}$Where I(t) is the intensity as a function of time.

In order to maintain the beam profile and divergence properties, theminors 20A-D that direct the beam through the delay propagation pathmust create an imaging relay system that also should act as a unity,magnification, focal telescope. The reason for this is because of theintrinsic divergence of the excimer laser beam. If the beam weredirected through a delay path without being imaged, the beam would be adifferent size than the original beam when it is recombined at the beamsplitter. To create the imaging relay and a focal telescope functions ofthe pulse stretcher 12 the mirrors 20A-D are designed with a specificradius of curvature which is determined by the length of the delay path.The separation between mirrors 20A and 20D is equal to the radius ofcurvature of the concave surfaces of the mirrors 20A-D and is equal to ¼the total delay path.

The relative intensities of the first two peaks in the stretched pulsecan be modified with the design of the reflectivity of the beam splitter16. Also, the design of the beam splitter 16 and therefore the outputt_(IS) of the pulse stretcher 12 are dependent upon the efficiency ofthe beam relay system and Therefore the output t_(IS) is also subject tothe amount of reflectivity of the imaging relay mirrors 20A-D and theamount of loss at the beam splitter 16. For an imaging relay mirrorreflectivity of 97% and a loss of 2% at the beam splitter 16, themaximum t_(IS) magnification occurs when the reflectivity of the beamsplitter 16 is 63%.

The alignment of the pulse stretcher 12 requires that two of the fourimaging relay mirrors 20A-D be adjustable. Each of the two adjustablemirrors would have tip/tilt adjustment creating a total of four degreesof freedom. It is necessary that the two adjustable mirrors be locatedat opposite ends of the system because of the confocal design of thesystem. To create a self-aligning pulse stretcher would requireautomated adjustment of the necessary four degrees of freedom and adiagnostic system which could provide feedback information tocharacterize the alignment. The design of such a diagnostic system,which could qualify the alignment performance, would require an imagingsystem capable of imaging both the near field and far field output ofthe pulse stretcher 12. By examining the overlay of the sub-pulses withthe original pulse at two planes (near field and far field) one wouldhave the necessary information to automatically adjust the mirrors toproduce an output where each of the sub-pulses propagate in a co-linearmanner with the original pulse.

Beam Delivery Unit

In this preferred embodiment a pulsed laser beam meeting requirementsspecified for the scanner machine 2 is furnished at the light input portof the scanner. A beam analysis module as shown at 38 in FIG. 1 called aBAM is provided at the input port of the scanner to monitor the incomingbeam and provide feedback signals to the laser control system to assurethat the light provided to the scanner is at the desired intensity,wavelength, bandwidth, and complies with all quality requirements suchas dose and wavelength stability. Wavelength, bandwidth and pulse energyare monitored by meteorology equipment in the beam analysis module 38 ona pulse to pulse basis at pulse rates up to 4,000 Hz using techniquesdescribed in U.S. patent application Ser. No. 10/012,002 which has beenincorporated herein by reference.

Other beam parameters may also be monitored at any desired frequency.Parameters such as polarization, profile, beam size and beam pointingare relatively stable so users may choose to monitor these parametersmuch less frequently than the wavelength, bandwidth and pulse energyparameters.

Beam Pointing Control

This particular BDU comprises two beam-pointing mirrors 40A and 40B oneor both of which may be controlled to provide tip and tilt correctionfor variations in beam pointing. Beam pointing may be monitored in theBAM 38 providing feedback control of the pointing of one or both of thepointing mirrors. In a preferred embodiment piezoelectric drivers areprovided to provide pointing response of less than 7 milliseconds.

A preferred beam pointing control technique can be described byreference to FIG. 10A. A beam analysis module (BAM) 38 is located at theBDU exit. Module 38 has sensors 38A that measure the beam pointing andposition errors as they enter the scanner. The error signals are sent toa stabilization controller 39 located adjacent to module 38 thatprocesses the raw sensor data and generates commands to drive faststeering turning mirrors 40A and 40B. These two fast steering turningmirrors 40A-B, each with 2 axes of control, are placed upstream of thebeam BAM 38. The turning mirrors 40A-B are each mounted to a faststeering motor. The motor actuates the mirror angle in two axes and thusredirects the path of the laser beam. Two motors with 2 axes of controlenable the BDU stabilization controller to independently regulate thevertical and horizontal beam pointing and position errors. The controlsystem corrects for the beam errors from pulse-to-pulse. Namely, thebeam errors from each laser pulse are fed to a feedback control systemto generate commands for the steering motors. The electronics used torun the feedback control system are located in the StabilizationController module 39. This BDU also includes two static turning mirrormodules 40C and 40D, a beam expander module 41 and a beam intensityattenuator module 43.

The vertical and horizontal beam pointing and position errors areevaluated at the BDU exit for every pulse of light generated by thelaser. In total there are four independent sensor measurements.

-   -   1. Vertical pointing error    -   2. Horizontal point error    -   3. Vertical position error    -   4. Horizontal position error

The BAM 38 (a Stabilization Metrology Module, “SMM”) as shown in detailin FIG. 10B contains the sensors and associated optics needed to measurethe pointing, position, end energy of the beam at the exit of the BDU(the entrance of the scanner.) Most of the beam energy passes throughmodule 38 for delivery to the scanner, while a small fraction isdiverted for the various measurements;

-   -   Pulse-to-pulse evaluation of beam pointing and position errors    -   Vertical and horizontal pointing is measured by putting        far-field images on linear photodiode array (PDA) elements, such        the S903 NMOS Linear Image Sensors offered by Hamamatsu        Corporation with offices in Bridgewater, N.J.    -   Vertical and horizontal position is measured by putting reduced        images of the beam near the BDU exit on linear PDA elements.    -   Beam energy measurement    -   The energy of the beam delivered by the BDU to the scanner is        measured with calibrated photo-cell circuit.

Signals from the sensor in the Stabilization Metrology Module (“SMM”)are sent through electrical connectors to the Stabilization Controller39.

A Brewster window 60 allows 95% of the beam energy to pass on to thescanner, deflecting 5% into the body of module 38 for use by the beammetrology sensors. The light deflected by the main Brewster window formetrology is split again by another Brewster window 62; the deflectedlight, which has the same polarization mix as the light sent to thescanner, is focused by converging lens 64 on a photo-cell energy sensor66.

The remainder of the light not deflected by the PDM Brewster window 62is distributed among four linear PDA sensors 68A, B, C and D formeasuring vertical and horizontal beam position and pointing. To measureposition, two beams split off by a wedge 69A are sent through aconverging lens to form images of the beam on two of the PDA sensors 68Aand 68B. The lens and path lengths are such that the images formed are ½scale images of the cross-section of the beam at the main Brewsterwindow. The two PDA sensors are oriented at 90° to one another so thatone measures the intensity profile of the beam in the verticaldirection, and the other measures the intensity profile in thehorizontal direction. Changes in the position of the beam at theBrewster window thus produce shifts in the reduced profile images on thesensors.

The light not deflected for the position sensors is passed throughanother converging lens 69C and wedge 69B so as to form spots on theremaining two PDA sensors 68C and 68D which are also oriented at 90° toone another. In this case, the PDS sensors lie in the focal plane of thelens 69C, so that changes in the pointing angle of the beam produceshifts in the positions of the spots on the sensors.

Mechanical shields 70A and 7B are placed in front of all the PDA sensorsto ensure that they detect only the intended light intensitydistributions.

Finally, a beam dump 72 dissipates any remaining light energy. This beamdump is removable to expose a window that may be used for diagnostics.

Because of the large range of delivered light intensity, a variableattenuator 74 is used upstream of the PDA elements to prevent them fromsaturating. The variable attenuator is a motorized device that placesvarious neutral density filters in the beam path, for example a versionof a motorized flipper model 8892 offered by New Focus with offices inSan Jose, Calif. The variable attenuator comprises an energy sensor anda feedback circuit and is motorized to automatically adjust the lightintensity arriving at the PDA elements. The attenuator setting isadjusted by feeding the energy sensor data to the stabilizationcontroller. An algorithm on the stabilization controller adjusts theattentuator setting based on the energy sensor reading. In oneembodiment, only one filter is used. When the energy setting is above apre-specified threshold, the filter is placed in the beam path toattenuate the energy of the beam. When the light energy drops below thepre-specified threshold, the filter is removed from the path. In otherembodiments, several filters may be required depending on the intensityrange of the light and sensor electronics dynamic range.

FIGS. 10C and 10D1-3 illustrate the signal processing performed togenerate pointing error measurements from the PDA detectors. Metrologyin module 38 places the vertical and horizontal far field spots on PDAelements. FIG. 10C illustrates a situation where the metrology rotatesone reflection of the beam so that both the vertical and horizontalspots are placed on the same PDA element.

Pointing errors are defined from target locations defined at the exit ofmodule 38. In other words, the laser user dictates where it wants thebeam leaving module 38. Module 38 is a compact, light weight unit thatcan readily be mounted at the beam entrance to scanner 2. Total modulesize and weight can be kept to within 50×25×15 cm and about 15 kg.

To compute pointing errors, a reference location on the PDA elements isspecified. The corresponding reference points on the PDA elements aredefined with respect to the scanner specified reference location.Namely, the metrology inside the module 38 is aligned so that zeropointing error corresponds to the center of the flit field spot fallingat the reference pixel location. On FIG. 10C the reference pixellocations are denoted by r_(v) and r_(h) for the vertical and horizontalfringes respectively.

The position of the far field fringes with respect to the referencelocation on the PDA elements reflects the pointing angle of the beam asit leaves the BDU. Likewise, the relative position of the image profileswith respect to the reference location on the PDA elements reflects theposition of the beam leaving module 38. The position of a far field spotor profile on a PDA shall be defined in terms of threshold crossings.(Alternately, the position could be defined in terms of the location ofthe centroid of the intensity distribution.) For each pulse, the firstand last pixels to exceed the threshold value (e.g. 1/e² of the maximum)are found, and the threshold crossing itself is determined byinterpolation with the neighboring pixels as illustrated in FIGS. 10D1,2 and 3. The midpoint between the threshold crossing is taken to be thecenter of the fringe (C_(v) and C_(h) representing the vertical andhorizontal center) and the error signal is the distance between thecenter of the fringe and the reference locations, (i.e., r_(v) andr_(h)). For example, the vertical pointing error is directlyproportional to the distance between rv and cv as shown in FIG. 10C.

Test Results

A prototype BDU system was actually built and tested by Applicants. Testresults at 2 KHz and 4 KHz are shown in FIGS. 10E and 10F with thepointing control on and pointing control off. In the open loop the beamstabilization system is off, and the steering mirrors are fixed. Thebeam from the laser propagates directly to the scanner withoutcorrection. The open loop errors are exactly the pointing and positionerrors generated by the laser. The closed loop behavior indicates theperformance achieved when the beam stabilization system is running.

FIG. 10E illustrates the vertical pointing performance achieved inApplicants' KrF experiments. Applicants plotted the moving average ofthe vertical beam angle measured with and without the activestabilization control as the repetition rate is changed. The readershould note that the changes in beam angle offset that accompany achange in repetition rate are eliminated, as is the variation in anglethat occurs over hundreds or thousands of shots at a constant repetitionrate.

FIG. 10F shows moving averages of horizontal and vertical beam anglescontrolled simultaneously for 200 pulse bursts of pulses with 0.5 secondintervals between bursts. As shown in FIG. 10F the vertical beam angleerror is reduced more than a factor of 10.

In FIG. 10G the actual measured angle for each shot in a burst ispresented. The pointing angle change at the beginning of a burst is thesame in both cases; but when the sensor measures a significant angleerror, the controller determines the proper command to send to theactuator, which quickly corrects the beam angle to near zero. The resultis a moving average performance as shown by dark lines 81A and 81B thatis greatly reduced from the uncontrolled case.

In FIG. 10H the same laser is used with the sensor equipment arranged tomeasure beam position rather than beam angle.

Fixed Energy Output

In general all optics in the beam path from the gain medium to thesilicon wafer degrade over time generally as a function of intensity ofthe light in each pulse and the number of pulses. However, because ofmajor improvements over the past few years that degradation is slow andis typically measured in billions of pulses. Still, the degradation issignificant since, at 4000 Hz, a round-the-clock operation at a 15percent duty factor, a lithography system will accumulate a billionpulses in a about three weeks. For this reason maintaining constant beamquality can be a challenge. In the past this effort to maintainconsistent beam quality over the life of the components of thelithography system has been complicated by the fact that laser beamquality for most laser control functions was measured at the output ofthe laser system, just downstream from the output coupler. The presentinvention greatly moderates this problem by providing directpulse-to-pulse feedback control at the input port of the scanner machineand by supplying the beam delivery unit as a part of the laser system.In this preferred embodiment the beam delivery unit is combined with theabove described MOPA system which produces approximately twice the pulseenergy as the current state-of-the-art lithography light sources with areduction in energy intensity and with substantial improvements in beamquality. Therefore, with this arrangement the present invention providesillumination meeting the requirements of the operator of the steppermachine with beam quality and intensity unchanged over the lifetime ofthe lithography system despite substantial degradation of opticalcomponents throughout the length of the beam path. This can beaccomplished by intentionally operating the laser system to provide adesired nominal performance at all stages of equipment life. Techniquesfor intentionally decreasing pulse energy include the usual technique ofreducing discharge voltage but also reducing gas pressure or fluorineconcentration. Beam attenuation is another possibility. This means thatin the early stages of equipment life when all components are new, thelaser may be operated so as to produce illumination with less thanoptimum quality and intensity, but quality and intensity values can bemaintained constant (if desired) throughout the life of the lithographysystem. This approach can substantially increase the useful life notonly of the very expensive laser system but also the much more expensivestepper machine. FIG. 5 is a plot of charging voltage vs. pulse energyoutput for a prototype MOPA laser system built and tested by Applicants.This chart shows that the laser system output can be varied betweenabout 7 mJ to 30 mJ merely by changing the charging voltage. Forexample, if a nominal operating parameter is 15 mJ, the graph in FIG. 5demonstrates that there is plenty of excess capacity in the laser tocompensate for optics degradation over a long equipment lifetime. Sincethe MOPA output in this embodiment is 30 mJ per pulse compared topresent state-of-the-art laser systems with output of 10 mJ, majorlifetime improvements are expected using the above-described plan.

BDU-Part of Laser

Another advantage of providing the laser beam at the entrance port ofthe scanner is that the beam delivery unit now becomes theresponsibility of the laser supplier for not only design and manufacturebut also for pro-active preventative maintenance so as to minimizedowntime and increase system availability.

Various Laser-BDU-Scanner Configuration

Another advantage is that the beam delivery unit can be designed as partof the laser system to suit the location of the laser with respect tothe lithography machine. FIG. 1 shows a typical configuration but mostlithography installations are unique and many other configurations areexpected to be utilized. Some of the various possible laser—BDU—scannerconfigurations are shown in FIGS. 4A, 4B, 4C and 4D.

Attenuator

In a preferred embodiment a special attenuator is included in the beamdelivery unit which provides controlled attenuation of the beam anywherewithin a range of 3 percent transmission to 90 percent transmission. Theattenuator could be located anywhere convenient in the beam deliveryunit 6. Preferably it is provided as a modular unit which can be boltedin place in the purged beam line.

The attenuator unit is comprised of two sets of two wedges 600A, 600B,602A and 602B as shown in FIG. 17A which are pivoted in oppositedirections with a worm gear arrangement as shown at 604 in FIG. 17B. Thewedge sets are mounted on shafts 604A and 604B. A magnet 606 attached toworm shalt 605 causes reed switch 608 to close once on each revolutionof shaft 605 as shown in FIG. 17C so that a signal is sent to controlunit 610 so that the radial positions of the wedge sets 600A and B and602A and B are known.

Light enters from the left side of the FIG. 17A as shown at 612 and hitsthe first set of wedges 600A and B. Depending on the angle of incidenceon the surface, light is partially reflected and partially transmittedoff each surface. Thus, the first wedge reduces the amount of lighttransmitted and also bends the transmitted beam. The second wedge, whilealso further reducing the amount of transmitted light, corrects thepointing of the beam due to its equal and opposite wedge angle withrespect to the first wedge. At this point, the exiting beam is muchweaker, shifted with respect to the entering beam, and parallel to it.The second set of wedges 602A and B performs an equal and oppositegeometric operation to the beam, thus ensuring that it exits parallel toand in line with the entering beam.

The concept relies on the matching of the wedge angles in each pair ofwedges to avoid beam pointing shifts at the output, and relies on equaland opposite angles between the two wedge assemblies to avoid positionshifts.

The total transmitted power is reduced due to the deflection of largepart of the beam; when the incident angle of the first surface of thewedges is equal to the Brewster angle for the particular wavelength oflight and the particular material chosen for the wedges, then most ofthe light (a total of over 92% of the power) is transmitted through theattenuator assembly. When the incident angle is shallower, the outputbeam power can be regulated, by changing the incident angle, the exitbeam power can be reduced down to less than 3% of the incoming beampower.

A second effect of this design relates to the polarization of the outputbeam. Due to the fact that the wedge assemblies are aligned with oneorientation of the beam, but are angled with respect to the other, thes- and p-polarizations of the incoming beam are affected differently bythis assembly. Through correct alignment of the device, the effect canbe a cleanup of the polarization. For example, typical Excimer Laserbeams are up to about 98% p-polarized, and it is desirable to have thispolarization number as high as possible after the attenuator system.With the design presented above, the p-polarized component of the beamis preserved, while the s-polarized component is reduced, leading to anet effect of increasing the p-polarization of the output beam.

Positioning of BDU Optical Modules

Traditional optical alignment techniques for aligning optical componentswithin optical modules involve sighting directly down the optical path,using an optical telescope or similar tool to align the components.Applicants have developed a technique that allows optical components tobe aligned without breaking into the beam path. The optical componentswithin the optical modules are precisely aligned with respect to somereference points or targets on the outside surfaces of the module.Optical modules are fitted with reference targets and the opticalmodules are precisely positioned with an accuracy of about 0.25 mmaccording to a previously developed optics layout using a precisionsurvey instrument such as a Total Station type survey instrument orother type of theodolites.

Visually accessible reference points on the modules must be preciselyaligned to a known axis or other feature of the optical component orcomponents within the module. If the optical path is contained withintubes, boxes or other geometry, then the reference points should lie onexternal surfaces of these containers. A Total Station transit (forexample) can then be used to align the external reference points to someknown design location, obtained from a model of the optical components.Three reference points are required to define the position and rotationof each optical module or other container of optical components.

See FIG. 12 as a general illustration of the concept. In this figure, aTotal Station 399 is used to align a generic set of optical components(each housed in a separate container) along an optical path 397. Notethat each of the external reference targets 395 on optical modules 393must be pre-aligned to the internal optical components. Generally thisis performed during the assembly of the components in the module.

FIGS. 12 and 12A illustrate the methodology as applied on a preferredBDU. A summary of the methodology is as follows:

-   -   1. First, the Total Station 399 is used to locate the optical        exit location of a laser. This location is recorded using the        Total Station and defined as point (0,0,0) in a global        coordinate system, as shown at 400 in FIG. 12A.    -   2. The locations of three or more fixed reference targets are        located and measured using the Total Station. These fixed        reference targets must be permanent immovable targets in the        room (typically directly on a wall, floor or ceiling), as shown        at 402A, 402B and 402C in FIG. 12A. Once the reference target        locations are recorded, the Total Station can be moved and        re-oriented as required.    -   3. The positions of target points on the modules requiring        alignment are imported from a 3D CAD model of the system. The        origin of the model (i.e. point (0,0,0)) should correspond to        the optical exit location of the laser as defined in step 1.    -   4. The actual positions in space of the module target points are        measured with the Total Station, as shown in FIG. 12B.    -   5. The measured position of each target point (from step 4) is        compared to the design position of the corresponding point (from        step 3) using the Total Station software.    -   6. Any difference between the measured and desired location is        corrected by moving the module the distance indicated by the        Total Station software.    -   7. Steps 4 through 6 are repeated until the module is aligned to        within a pre-determined accuracy (typically 0.25 mm).

A number of tools used to perform the procedure above are shown in FIGS.12C, 12D, 12E and 12F. FIG. 12C shows two views each of three precisionmounting targets 404 used to align the modules. The targets areavailable from suppliers of survey equipment in these threeconfigurations to allow viewing of the targets from various angles. Thetargets can be inserted in precision drilled reference holes on eachmodule (preferably four or five holes are provided on each of fourfaces). FIG. 12D shows a set of the targets 404 inserted in theprecision holes in module 405. A total of three target points are neededto align each module. The extra holes allow flexibility in targetplacement, as each of the three targets must be visible from the TotalStation.

After a module 405 has been aligned, it may be necessary to remove themodule for service or replacement. Rather than using the Total Stationto re-align the module, a device that marks the position of the moduleis desirable. FIG. 12E shows a “Memory Device” 406 that is used tolocate the position of a module alter it has been aligned. It isbasically a metal part with two bolt slots 408 and a “c” shaped gap. Thec-shaped gap is snugly fitted around a corner of an optics module andthen the Memory Device is firmly bolted to an alignment plate 411 onwhich the optics module sets with bolts 412 as shown in FIG. 12F. Byaffixing two of the Memory Devices around each of two corners of amodule, the location of the module is fully defined. If a module isremoved for maintenance a replacement module having optical componentslocated identically to the ones in the replaced module, the replacementmodule can be placed in the exact same position as that previouslyoccupied by the replaced module without any manual alignment. In thiscase the alignment ranges of the automatic alignment componentsdiscussed above are large enough so that the system automaticallycompletes the final steps of precise alignment.

Polarization Considerations

In the master oscillator 8 resonant cavity optical components includingtwo windows and three prisms are oriented with surfaces orientedvertically providing several angles of incidence, with the developinglaser beam, close to Brewster's angle. Therefore, beam 14A exiting themaster oscillator 8 is strongly polarized with about 98 percent of theelectric field component of the beam being in the horizontal directionand about 2 percent being in the vertical direction.

When using dielectric coated mirrors at 45 degrees for beam turning, itis important to take into consideration polarization effects becausewith these mirrors S-polarization is reflected nearly 97 percent whereasP-polarization is reflected only 90 to 92 percent. (P-polarizationrefers to the electric field component of the light which is in theplane defined by the beam direction and a line perpendicular to theoptical surface at the intersection of the beam direction and thesurface. S-Polarization refers to the direction of the electriccomponents of the light in the plane of the surface and perpendicular tothe P-polarization). Therefore, to maximize reflection from turningmirrors, it is important that the S-polarization direction correspondsto the polarization of the incoming beam. As the reader will note minors40A and 40B are both oriented so that the S-polarization direction ishorizontal corresponding to the electric field direction of about 98percent of the light in output beam 14C; therefore reflection should beabout 97 percent from these mirrors. The mirror shown in the BDU shownin FIGS. 4A, 4B and 4C are all properly for oriented maximum reflectionof horizontally polarized light However, the mirror shown at 52 in FIG.4D is oriented so that the P-polarization direction is in the directionof the electric field direction of 98 percent of the light in the beamso that reflection by this mirror would be only about 90 to 92 percent.In this ease Applicants preferred solution is to utilize two prisms tomake the 90-degree beam turn at the 50 location in FIG. 4D. Thistechnique is shown in FIG. 6. Two prisms 52 and 54 with an apex angle of67.2-degrees (the angle is important) can change the angle of incidenceby 90 degrees for the s-polarized light. The beam enters and exits theprism at Brewster angle, so there is no reflection at all of light inthe horizontal direction. The portion of the beam polarized in thevertical direction would be mostly reflected by the first prism. Thelayout is done for 193 nm and CaF2 prisms. (Minor modifications would beneeded for 248 nm or 157 nm). Since no coatings are involved, thelifetime of this assembly is very high.

As the horizontal polarized light passes through the two prisms atlocation 50 in FIG. 4D the direction of polarization of substantiallyall of the electric field components is reoriented from horizontal tovertical as indicated by arrows 53A and 53B as shown in FIG. 6.

Purge Shutters for Mirrors

The BDU volume could be large, as much as 200 liters and must be purgedwith high purity N₂. This purging process may take several hours to getto the free ppm level of oxygen and other organics. During the firstinstallation of the BDU to the scanner, this purge time is acceptable,but is considered very long during normal operation. Assume that amirror, such as mirror 60 in FIG. 4A needs service. This may entaildismantling the mirror from BDU which could expose the BDU to air.Hence, what could be a brief service procedure (replacing the minor)turns Into a very long purge procedure. To avoid substantial delaysassociated with a long purge period to restore the quality of the beampath in the BDU, BDU shutter units 62 are added on both sides of eachmirror in the BDU as shown in FIG. 7 for mirror 60.

Here, In the BDU are located several inserts where service shutters maybe inserted to isolate the other regions in a BDU. These shutters arenormally not inserted during operation. For example, as shown in FIG. 7,two shutters are slid around mirror 40A that needs to be isolated andthe rest of the BDU and then the mirror itself is replaced. After that,this exposed region is now purged with N₂ for a few minutes. The purginginterval is much shorter now due to the fact that the volume exposed toair is much smaller than the total volume of the BDU. Preferably duringthe servicing purging continues in all regions of the beam path otherthan that between the shutters.

Beam Path Purge

In this preferred embodiment all portions of the beam path outside thelaser chambers are purged with N₂, with two exceptions: (1) The linenarrowing package and the portion of the path between laser chamber 8Cand the LNP is purged with helium and (2) the etalon chambers in theLAM, SAM and BAM for measuring wavelength and bandwidth are sealedchambers. FIG. 1 shows a purge gas supply at 42 but the purge lines arenot shown. Excellent examples of purged beam paths are described indetail in U.S. patent application Ser. No. 10/000,991 which isincorporated by reference herein. This technique includes metal bellowsand easy sealing vacuum quality seals at interfaces between thevibrating chambers and the sensitive laser optics and vacuum qualityseals at interface between all separate modules permitting quickseparation of the modules to permit fast module removal for maintenanceor for service. FIGS. 8A through B show drawings of preferred easysealing bellows seals units with parts 93A, B and C useful for makingconnection for components in the beam path from the LNP to the scanner.Either of the clamps shown in FIGS. 8C and 8E can be used to clamp parts93A and 93B together with the Un coated metal C-seal sandwiched inbetween. FIG. 8D shows a cut-away of the assembled seal unit. The sealsin the seal units are metal “C” seals preferably with a tin containlayer. The metal seals do not deteriorate or allow out gas contaminationunder ultraviolet irradiator.

Beam Path Monitor

Preferably monitors are provided to assure the quality of the laser beampath since contamination of the path with absorbers such as oxygen cansubstantially affect beam quality and pulse energy. Preferably severalpurge paths will be provided. Flow monitors can be used to monitor purgeflow; however, other monitors may also be provided such as O₂ monitorswhich are commercially available from several suppliers.

Another beam path quality monitor includes an acoustic monitor utilizinga electret electronic microphone available from suppliers such as AudioProducts with offices In Dayton, Ohio. This type of monitor is describedin U.S. Pat. No. 10/000,991 which is incorporated by reference herein.In preferred embodiments these monitors are used to provide signalswhich may be used by the lithography operator to delay fabrication aftera shutdown until the beam path purge has sufficiently cleared the beampath of contamination.

Beam Profile Flipping

For integrated circuit fabrication coherence of the laser beam isundesirable. Excimer laser beams characteristically have poor coherencewhich is one of the many reasons why this light source is good forintegrated circuit fabrication. However, as other aspects of the beamquality continue to get better, even the poor coherence of the laserbeams from these lasers may not be poor enough. If that turns out to bethe case a coherence scrambler can be added. It could be added atseveral places in the beam path. A good place for it would be anywherein the beam delivery unit.

FIG. 9 shows an example of a beam profile flipping coherence scrambler.This is produced with a 60 percent beam splitter 60 and three maximumreflection mirrors 62, 64, and 66. This arrangement segregates the pulseinto segments in a manner similar to the pulse stretcher discussedabove. But with this configuration the profile of each segment isflipped with respect to the preceding segment. In the FIG. 9 example theprofile of the incoming pulse 68 is represented with a triangle with apoint at the bottom. The first segment, 40% of the pulse intensitypasses through with the same profile as shown at 68A. The reflectedportion suffers reflection at each of the minors and 60 percent of it isreflected from beam splitter 60 that segment has a profile shown at 68Bwhich is flipped with respect to profile 68A. As subsequent segmentspass through the coherence scrambler their profiles are each flippedwith respect to the preceding segment. Thus, the net profile of the beamwill be scrambled and more importantly any coherence will also bescrambled. The reader should note that in this embodiment there will beno significant pulse stretching unless the legs are long enough toprovide significant delays of the segments following the first one.Since we have already stretch the pulse as described above the legs herecould be very short such as a few inches in which case the segments willoverlap each other.

Pulse Energy Detection at Wafer Plane

In preferred embodiments of the present invention as shown in FIG. 1 apulse energy detector 44 is provided at wafer plane 46 in the scanner.Pulse energy signals this detector may be used in a feed back loop tocontrol the energy output of the laser directly. Alternatively, thesignals may be used for the purpose of determining pulse energyparameters as measured at the BAM or the SAM which will provide theillumination needed at the wafer plane.

Optics Monitor

Preferred embodiments of the present invention produce pulse energiesapproximately twice as large or greater than state-of-the-artlithography lasers currently in use. Repetition rates are at least asgreat or greater than this state-of the-art-lasers. These pulse energiesand repetition rates pose potential danger to optical components such asmirrors, lenses and prism used in the laser system and downstream of thelaser. When and if these components fail they adversely affect beamquality. However, with many optical components in the beam, finding thedeteriorated optic may be difficult. A preferred solution to this issueis to attach a thermocouple to the optical components to permit easymonitoring of the temperature of the component.

The signals from the thermocouple may be read periodically by a dataacquisition computer which may be programmed to provide a warning iftemperatures exceed a predetermined threshold. A preferred technique formonitoring mirrors is to attach the thermocouple to the back of themirror with solder or an eproxi. The thermocouple may be attached to theedge of lenses and prisms or to the lens or prism mounts.

Special F₂ Laser Features

The above descriptions generally apply directly to an ArF laser systembut almost all of the features are equally applicable to KrF lasers withminor modifications which are well known in the industry. Somesignificant modifications are required, however, for the F₂ version ofthis invention. These changes could include a line selector in the placeof the LNP and/or a line selector between the two chambers or evendownstream of the power amplifier. Line selectors preferably are afamily of prisms. Transparent plates properly oriented with respect tothe beam could be used between the chambers to improve the polarizationof the output beam. A diffuser could be added between the chambers toreduce the coherence of the output beam.

Prototype Beam Delivery Unit

A production ready prototype beam delivery unit built and tested byApplicants is shown in FIG. 11A. The output of a laser system enters theBDU at location shown at 300 and the BDU delivers the beam to a steppermachine at location shown at 302 FIG. 11B. The beam path is completelyenclosed and is purged with nitrogen. The unit includes metrology module38 and two high speed precision turning minors located at 40A and 40B.Beam stabilization controller is shown at 39. Another view of theprototype unit is shown in FIG. 11B.

One of the two high speed precision turning mirrors is shown in FIG.11C. A cut-a-way of this mirror unit is shown at 304. This mirrorincludes a very fast two-axis piezoelectric driver fast steering mirror305 with a one milliradian range. Mirror 305 and the base 306 on whichit operates is driven by pico motor steering unit 307 comprised of twopico motors 308 and 310 and a pivoting ball joint (not shown). The picomotor steering unit provides a tip-tilt turning range of 9 milliradians.

FIG. 11D is a drawing of the piezo driver unit 305A for driving faststeering mirror 305. The driver unit is comprised of four piezoelectricdrive units mounted inside metal casing 305B. A flecture feature 305Ccut in the walls of casing 305B permits tightly controlled precisepivoting of the mirror unit for mirror 305.

FIG. 11E shows pico motor steering unit 307 and FIG. 11F shows how thetwo pico motors pivot the unit to provide tip and tilt. The motorsoperate against spring units 309. Fast steering mirror 305 fits in thecircular cavity in unit 307. Fast steering units drivers as shown inFIG. 11C are available from suppliers such as Polytec PI, Inc. withoffices in Alburn, Mass.

The beam positions and beam directions are monitored by stabilizationmodule 38 at the input port of the stepper machine. Four 512-pixelphotodiode arrays are used to monitor the horizontal angle, verticalangle, horizontal position and vertical position. As shown in FIG. 11G,a portion of the laser beam is picked off at 312, reduced in size, splitinto four separate beams using wedge 314 and beam splitter 316 and thendirected to the four photo diode arrays 318A-D. The technique formonitoring the beam position and direction is described above in thesection entitled Beam Pointing Control. FIGS. 11H, I and J demonstrate apreferred algorithm for controlling mirrors 40A and 40B based on datacollected by the stabilization module 38.

In this preferred algorithm the turning mirror 40A is used to controlbeam position at the output of the beam delivery unit and turning mirror40B is used to control the beam angle at the output.

The fast steering mirror provides fast response and the pico motor unitcontrols long term drift and provides correction when optics arerealigned.

Test Data

Actual test data showing the excellent performance of this beam deliveryunit is shown in FIGS. 11K, 11L, 11M, 11N. FIG. 11K shows angle controlwith the control on and off. FIG. 11L shows position control. FIG. 11Mshows angle control at low output energy and FIG. 11N shows positioncontrol at low output energy. In all cases the controlled values aremaintained on target well within specifications shown by dashed lines,whereas the uncontrolled values are typically out of specifications.

Various modifications may be made to the present invention withoutaltering its scope. Those skilled in the art will recognize many otherpossible variations.

For example, although the invention, including the utilization of a beamdelivery unit, is described using a MOPA laser configuration, a singlechamber laser system such as described in U.S. Pat. No. 6,730,261 couldbe utilized. For lithography either ArF, KrF or F₂ systems could beutilized. This invention may also be applied to uses other thanlithography in which other ultraviolet wavelength may be moreappropriate. An important improvement here is the addition of equipmentto a laser system to deliver an ultraviolet laser beam having desirebeam qualities to an input port of a equipment needing an ultravioletlaser light source. Various feedback control arrangements other thanthose referred to herein could be used.

The reader should understand that at extremely high pulse rates thefeedback control on pulse energy does not necessarily have to be fastenough to control the pulse energy of a particular pulse using theimmediately preceding pulse. For example, control techniques could beprovided where measured pulse energy for a particular pulse is used inthe control of the second or third following pulse. Many other laserlayout configurations other than the one shown in FIG. 1 could be used.For example, the chambers could be mounted side-by-side or with the PAon the bottom. Also, the second laser unit could be configured as aslave oscillator by including an output coupler such as a partiallyreflecting mirror. Other variations are possible. Fans other than thetangential fans could be used. This may be required at repetition ratesmuch greater than 4 kHz. The fans and the heat exchanger could belocated outside the discharge chambers.

It may be desirable to include additional special features to protectoptics from damage due to the high intensity laser pulses. Some of thesefeatures (including the addition of F₂ or an F₂ containing substance inthe purge volumes) are described in detail in the parent applicationsreferred to in the first sentence of this application.

Accordingly, the above disclosure is not intended to be limiting and thescope of the invention, should be determined by the appended claims andtheir legal equivalents.

1. A beam delivery unit for directing a laser light beam optical outputof a laser light source to a light input location on a laser light usingstation, comprising: A. a beam delivery unit comprising a beam pathenclosure structure providing a laser beam path from a beam deliveryunit input positioned at the optical output of the laser light sourceand a beam delivery unit output at the light input location on the laserlight using station; B. a laser light beam attenuator means positionedin the beam delivery unit between the beam delivery unit input and thebeam delivery unit output for attenuating the amount of laser lighttransmitted through the laser light attenuator, the light beamattenuator means comprising two partial light transmitting means.
 2. Theapparatus of claim 1, further comprising: A. a first light transmittingmeans is adjustably positioned in the path of the laser light beam tochange the angle of incidence of the laser light beam on the firstpartial light transmitting means, and positioned to transmit a firstselected portion of the laser light beam responsive to the angle ofincidence of the laser light beam on the first partial lighttransmitting means; and, B. a second partial light transmitting means isadjustably positioned in the path of the laser light transmitted throughthe first partial light transmitting means and positioned to transmit asecond selected portion of the light in the laser light beam and torealign the laser light beam.
 3. The apparatus of claim 2, furthercomprising a positioning means for adjusting the position of the angleof incidence of the laser light beam on each of the first and secondpair of partial light transmitting means.
 4. The apparatus of claim 3,further comprising the positioning means included means for positioningthe first and second partial light transmitting means equal an oppositeangles of incidence of the laser light beam.
 5. A method of operating abeam delivery unit to direct a laser light beam optical output of alaser light source to a light input location on a laser light usingstation, comprising: A. positioning a beam delivery unit comprising abeam path enclosure structure providing a laser beam path from a beamdelivery unit input at the optical output of the laser light source anda beam delivery unit output at the light input location on the laserlight using station; B. attenuating the amount of laser light reachingthe beam delivery output by a preselected amount using a laser lightbeam attenuator positioned in the beam delivery unit between the beamdeliver unit input and the beam delivery unit output, using two partiallight transmitting elements.
 6. The method of claim 5, furthercomprising: A. positioning a first partial light transmitting element inthe path of the laser light beam to change the angle of incidence of thelaser light beam on the partial light transmitting element, and totransmit a first selected portion of the laser light beam responsive tothe angle of incidence of the laser light beam on the first partiallight transmitting element; and, B. positioning a second partial lighttransmitting element in the path of the laser light transmitted throughthe first partial light transmitting element to transmit a secondselected portion of the light in the laser light beam and to realign thelaser light beam.
 7. The method of claim 6, further comprising adjustingthe position of the angle of incidence of the laser light beam on eachof the first and second pair of partial light transmitting elementssimultaneously.
 8. The method of claim 7, further comprising positioningthe first and second partial light transmitting elements with equal anopposite angles of incidence of die laser light beam.